1. Technical Field
The present invention relates to hard alloy cutting tools having a multilayer surface coatings for providing good adhesion, wear and chipping resistance.
2. Technical Background
The application of coated hard alloys for insert cutting tools (referred to as inserts hereinbelow) has been gaining popularity in recent years. For disposable inserts, the percentage of the coated tools has reached about 40% in Japan, and more than 60% in western countries.
One reason for such popularity for the coated inserts is the improvement in the toughness of the substrate materials.
It is known generally that when the surface of hard alloys is protected with a hard coating, although the wear resistance is improved, the resistance to chipping is degraded. To rectify this problem, it is essential to improve the toughness of the substrate material. However, improving the toughness often means sacrificing the hardness which provides a basis of wear resistance but is in a converse relationship to toughness.
For this reason, the past solutions for improving the toughness of coated hard alloys involved mainly the surface layer portion of the substrate material, not the substrate material itself. The concept is that if the interior (core) of the hard alloys is hard, and the surface layers of the substrate material is tough, both wear resistance and chipping resistance can be improved simultaneously.
In fact, many of the coated hard alloy inserts in the markets for cutting steels and ductile cast irons, are made so that the surface layer is high in Co and has high toughness, and the core is relatively low in Co and has high hardness.
Such materials were first disclosed in a Japanese Patent Application, First Publication, Showa52(1977)-Laid Open No. 110,209, which disclosed a coated hard alloy of improved toughness as a result of having a surface layer thickness of 10-200 .mu.m, whose hardness is lowered by 2-20% compared with that of the core of the substrate material.
In this patent application, the first embodiment shows a substrate material of a composition, WC-10% TiC10% Co (by weight in all the subsequent cases, unless otherwise stated), coated with a slurry of WC-10% Co, dried and sintered at 1430.degree. C. for one hour to prepare a surface layer thickness of 130 .mu.m, Vicker's hardness of 1320 in the surface layer, and 1460 in the core. There are no TiC particles, which are brittle, in the surface layer and the volume percent of the Co phase in the surface layer is higher than that in the core. A chemical vapor deposited (CVD) TiC coating of a 6 .mu.m thickness is provided on the Co-enriched surface layer, thereby producing a coated high toughness hard alloy.
In the second embodiment, a TiC coated hard alloy is presented in which a mixture consisting of WC-6% Co and WC-10% Co is press compacted and sintered to produce a substrate material having a surface layer thickness of 80 .mu.m, and Vicker's hardness 1320, and a core Vicker's hardness of 1450.
In the meanwhile, a Japanese Patent Application, First Publication Showa53(1978)-131909, discloses in the claims, a coated hard alloy having a softer but tough surface layer, in which the hardness increases continuously towards the core.
In the first embodiment of the above-noted application, a method of preparing a substrate material from a powder mixture of WC-1% TiC-3% TaC-6% Co, by sintering at 1400.degree. C. for 30 minutes in a vacuum of 2.times.10.sup.-2 torr, depositing a Co surface layer of a thickness of 25 .mu.m, and sintering at 1430.degree. C. for 30 minutes in a 300 torr hydrogen. By such a process, a hard substrate material is obtained in which a Vicker's hardness gradient is present, from a value of 1050 at the external surface, 1260 at 15 .mu.m depth, 1520 at 60 .mu.m and 1540 at 500 .mu.m depths, and having a Co concentration which decreases towards the core from the surface which consists of only Co at the depth of 1-2 .mu.m. The surface of this substrate material is coated with a 5 .mu.m thickness CVD TiC, to produce a coated hard alloy.
In the second embodiment of the above-noted patent application, another example which involves the steps of preparing a mixture of WC-9% TiC-10% TaC-8% Co, sintering at 1450.degree. C. for 1 hour in a vacuum of 2.times.10.sup.-2 torr, coating the surface with graphite and sintering at 1450.degree. C. for 30 minutes, to produce a substrate material having a Vicker's hardness gradient which increases from a value of 1160 at the surface towards the core as, 1290 at 15 .mu.m depth, 1490 at 60 .mu.m depth and 1450 at 500 .mu.m depth. The surface of this substrate is coated with a CVD TiN coating of a 4 .mu.m thickness, to produce a coated hard alloy.
In a U.S. Pat. No. 4,277,283 (Japanese Patent Application, First Publication, Showa54(1979)-Laid Open No. 87,719, discloses in the claims, an example of a coated substrate material having high toughness surface layers of a 5-200 .mu.m thickness, in which the proportion of the B-1 type hard phases, TiC, TaC and TiN containing W, in the surface layer is lower compared with that in the core.
In the first embodiment of the above-noted patent, a sintered hard metal is disclosed, produced from a powder mixture consisting of WC-4%(Ti.sub.0.75 W.sub.0.25)(C.sub.0.68 N.sub.0.32)-5%(Ta.sub.0.75 Nb.sub.0.25)C-5.5% Co, heating the mixture in a 10.sup.-3 vacuum at 1450.degree. C. to eliminate B-1 type hard phase completely to a depth of 10 .mu.m, so that the surface layer is virtually all WC-Co. The surface of the substrate material is coated with a 6 .mu.m thick CVD TiC coating to produce a coated hard alloy cutting tool. The toughness of this tool is high because the surface layer becomes enriched with Co as the B-1 type hard phase is eliminated.
The second embodiment shows a substrate material made of a power mixture, WC-6.3%(Ti.sub.0.75 W.sub.0.25) (C.sub.0.68 N.sub.0.32)-7.5%(Ta0.75Nb.sub.0.25)C-10.5% Co, which is sintered at 1380.degree. C. in a vacuum of 10.sup.-3 torr, and depositing a 6 .mu.m thick coating of TiC, to produce a coated hard alloy. Other examples in the above-noted patent include a substrate material of a mixture WC-4%(Ti.sub.0.75 W.sub.0.25)(C.sub.0.68 N.sub.0.32)-5%(Ta.sub.0.75 Nb.sub.0.25)C 5.5% Co, which is heated at 1450.degree. C. in a vacuum of 10.sup.-3 to produce two types of substrate materials: a substrate in which free carbon particles are precipitated; and a substrate in which free carbon particles are not precipitated. The surfaces are coated with a 6 .mu.m thickness coating of TiC followed by 1 .mu.m thick Al.sub.2 O.sub.3 to produce coated cutting tools. Other examples concern materials of a general composition represented by (Ti, W)(C, N) and coating the surfaces with the usual CVD TiN coating to a thickness of 6 .mu.m.
Another U.S. Pat. No. 4,610,931, discloses hard alloy substrate materials containing no free carbon particles, and having no B-1 type phase in a surface layer (claim 1); having a Co-enriched surface and no B-1 phase in the surface layer (claim 6). These substrate materials are coated with coatings such as TiC, TiN and Al.sub.2 O.sub.3 by the usual CVD method.
However, when the B-1 type phases in the surface layer are eliminated, Co enrichment occurs simultaneously in the region, therefore, these hard alloys and coated hard alloys become identical with those disclosed in Japanese Patent Application, First Publication Showa54(1979)-Laid Open No. 87719.
The above-noted U.S. Pat. No. 4,610,931, discloses further: hard alloys containing no free carbon particles in which a part of the surface is removed by grinding and heattreated again to covert the nitrides and carbonitrides in the surface layer to carbides (claim 25); Co-enriched surface hard alloy (claim 30); and above-treated and coated hard alloys (claim 32).
The first embodiment of this patent shows a material WC-10.3% TaC-5.85% TiC-0.2% NbC-8.5% Co-1.5% TiN, which is heated at 1496.degree. C. for 30 minutes; sintered in a vacuum; made into a cutting insert after which the upper and lower surfaces (rake surfaces) are ground; heated again at 1427.degree. C. for 60 minutes in a vacuum at 100 .mu.m Hg, and after cooling at a given rate to 1204.degree. C., the flank surface is ground. The surface is coated with TiC and TiN coatings using the usual CVD coating method to produce coated hard alloys having no free Carbon particles, and having a Co-enriched layer and no B-1 type phases to a depth of 22.9 .mu.m, and coated with a multilayer consisting of 5 .mu.m thick TiC, 3 .mu.m thick TiCN and 1 .mu.m thick TiN layers.
In another U.S. Pat. No. 4,830,930 (corresponding Japanese Patent Application, First Publication Showa63(1988)-Laid Open No. 169,356), which discloses in the claims, a hard alloy substrate material, in which the surface layer of 10 to 500 .mu.m thickness contains a gradient of a binder phase (Co-containing phase) such that the binder phase concentration is maximum at the surface decreasing to a level at a depth of 5 .mu.m towards the core.
The first embodiment of the above-noted patent discloses a method of producing a substrate material following the steps of: preparing compacts of a powder mixture of WC-5% TiC-7% Co; sintering the compacts at 1380.degree. C. for one hour; carburizing at 1330.degree. C. for 10 minutes in an atmosphere of a 20 torr 80% H.sub.2 -20% CH.sub.4 mixture; decarburizing at 1310.degree. C. for 2 minutes in an atmosphere of 10 torr 90% H.sub.2 -10% CO.sub.2 mixture; cooling in a vacuum; thereby obtaining a microstructure having a Co content which is maximum at the surface and gradually decreases towards a core Co value. The substrate material thus prepared is coated with a CVD TiC coating of a 5 .mu.m thickness.
Other examples include substrate materials of a composition, WC-3% TiC-3% TaC-1% NbC-5% Co, treated by the same processing steps as above, and coated with TiC/TiCN/Al.sub.2 O.sub.3 coatings to provide coated hard alloys.
The foregoing extensive review of the prior art technologies is given to show that the studies are mostly concerned not with improving the coatings but with improving the toughness of the surface layer, which provided improved chipping resistance but which still left a problem of low wear resistance.
In the following section, research studies for improving the properties of the coatings will be reviewed. Representative examples are U.S. Pat. No. 4,497,874 and U.S. Pat. No. 4,812,370 (Japanese Patent Application, First Publication, Showa63(1988)-Laid Open No. 89666).
U.S. Pat. No. 4,497,874 discloses a coated hard alloy material having a Co-enriched surface on which a first coating of TiN is deposited. The reason recited for using the first layer of TiN instead of the usual coating of TiC is if TiC coating is applied directly to the Co-enriched surface layer, alloying occurs in the enriched layer. Therefore, the first TiN coating is used to prevent such alloying, and to form a thick layer of TiC directly on the TiN layer without resorting to forming a gradation layer.
In the first embodiment of the above-noted patent, a method is disclosed of preparing a substrate material of WC-6% TaC-6% Co-5%(W.sub.0.5 Ti.sub.0.5)C, according to the steps of: preparing pressed compacts and dewaxing at 1260.degree. C.; heating the dewaxed compacts in a partial vacuum of 600 torr and flowing nitrogen (at 3 L/min) for 45 minutes; removing the nitrogen and raising the temperature to 1445.degree. C. and sintering the compacts for 100 minutes; to produce a substrate material having a Co-enriched 30 .mu.m thick surface layer in which there is no B-1 type phase. The hard alloys are produced by coating the substrate material with TiN/TiC/TiN or with Al.sub.2 O.sub.3.
U.S. Pat. No. 4,812,370 (Japanese Patent Application, First Publication Shows63(1988)-89666) discloses in the claims, a coated hard alloy having a Co-enriched surface layer on which WC and a Co-diffused TiC first coating is deposited, a TiCN-TiN second coating to prevent the diffusion of WC and Co, a third coating of pure TiC, and a fourth coating, such as TiCO, TiCNO and Al.sub.2 O.sub.3.
The preferred embodiments of the above-noted application disclose, a coated hard alloy material of WC-12.4%(Ti.sub.0.46 Ta.sub.0.22 W.sub.0.32)(C.sub.0.80 N.sub.0.20)-8.0% Co, having a Co-enriched surface layer of an 18 .mu.m thickness, and having a 3 .mu.m thick TiC coating with diffused WC and Co, a 2 .mu.m TiCN coating, a 2 .mu.m TiC coating and a 0.3 .mu.m Al.sub.2 O.sub.3 coating.
The foregoing technologies are aimed at solving the problems of chipping of hard alloys when a CVD coating is applied directly to the Co-enriched surface layer of a substrate material, causing the formation of undesirable microstructures such as pores and a brittle eta phase in the surface layer, due to the diffusion of WC and Co from the substrate. The TiC coatings with diffused WC and Co suffer also from poor wear resistance.
The hard alloy produced according to U.S. Pat. No. 4,497,874 still present problems such as the poor adhesion of the first coating TiN to the substrate material, and inadequate wear resistance because the primary coating is TiC. Also, the step of decarburizing disclosed (in claims 11, 12 and 15) before the first coating of TiN is applied to the substrate material, is not effective for improving the wear resistance significantly.
The technology disclosed in U.S. Pat. No. 4,812,370 (Japanese Patent Application, Showa63(1988)-89666) is also deficient in that the wear resistance is inadequate because of inter-diffusion of WC and Co from the surface layer into the first TiC coating, and because of the poor adhesion between the first coating TiC and the second coating TiCN.
To rectify such problems in the existing coated hard alloys as outlined above, the present invention presents a new technology for preparing a coated hard alloy cutting tool of high toughness and high resistance to wear and chipping, and whose Co-enriched surface layer is free of detrimental microstructures, such as pores and brittle phases (an eta phase in the embodiments). The coatings are made to adhere tightly to the substrate material by controlling the Co distribution in the Co-enriched surface layer, and by adopting a new surface coating technique.